This invention relates to amplifiers, and more particularly to an output following cascode circuit placed between a voltage follower and an input signal source.
Most audio and operational amplifiers employ Miller-effect compensation for stabilization. In some circuits, Miller-effect compensation may not be readily apparent, but if internal device capacitances are taken into consideration some form of Miller-effect compensation is apparent. The Miller effect of a device is the increase in the effective base-to-collector capacitance of a transistor, or grid-to-cathode capacitance of a vacuum tube, because the collector, or plate, induces a charge on the base, or grid, through internal capacitance. This Miller effect may also be observed in field-effect transistors.
The advantage of Miller-effect compensation is that the pole in the response of the stage around which the compensation is present produces a zero at the same frequency in the Miller-effect impedance. This results in a single pole response, simplifying compensation. To take advantage of this compensation, additional capacitance is added between the collector and base of a transistor in audio power amplifiers and operational amplifiers, examples of which are analyzed by Edward M. Cherry in companion papers titled "Feedback, Sensitivity, and Stability of Audio Power Amplifiers" and "Nested Differentiating Feedback Loops in Simple Audio Power Amplifiers" published in the Journal of Audio Engineering Society, Vol. 30, No. 5, 1982.
A typical audio amplifier or operational amplifier is comprised of three stages: an input stage, an intermediate stage and an output stage. The input stage is a transconductance amplifier, sometimes with an active current sink. The intermediate stage is an inverting amplifier providing both current and voltage gain. It is usually the intermediate stage that is compensated by the Miller-effect capacitance, sometimes augmented by external capacitance, and an input or output lag capacitor. The output stage is a voltage follower providing the current required by the load. It has been discovered that over the frequency range of 1 to 100 KHz there is a significant distortion of about 0.2% or more in the output stage of a typical amplifier with a distortion for the whole amplifier increasing from about 0.003% to more than 0.2% as frequency increases.
The degree of distortion can be determined by assuming a sinusoidal voltage and current at the output, and computing the voltage and current, and the harmonics, at each node connecting the stages. By starting at the output and working towards the input, the required input voltage and current, and harmonics, can then be easily computed. The percentage of distortion can be computed by scaling these by the reciprocal of the feedback network's transfer function and comparing to the output voltage. This analysis ignores common mode rejection problems in the input stage and interaction with nonzero impedances connected to the input. Minimizing the input voltage swing by connecting the amplifier in an inverting configuration or by using cascode circuitry in the input can minimize common-mode nonlinearity problems.
In Cherry's study of the feedback, sensitivity and stability of audio power amplifiers (supra), three-dimensional distortion mechanisms are revealed: input stage transconductance nonlinearity, and limiting, resulting in transient intermodulation (TIM) distortion; output stage current nonlinearity; and output stage voltage nonlinearity. The effect of input stage nonlinearity can be minimized by reducing the ratio of the peak change in current to the static bias current. This is related to the slew rate of the amplifier.
The sensitivity of the amplifier to changes in output stage current and current gain shows strong dependence on the nominal current gain of the intermediate and output stages. The sensitivity of the amplifier to output current nonlinearities (such as are produced by loudspeakers operating near resonance) and to current gain nonlinearities can be arbitrarily reduced by increasing the current gain of the intermediate and output stages. MOSFETs used in power amplifiers require substantial currents to charge their internal device capacitances and produce a frequency-dependent current for these devices. Bipolar power transistors also require additional current at higher frequencies to charge their internal device capacitances.
According to Cherry, the sensitivity of the amplifier to changes in output stage transconductance or voltage nonlinearities is dependent on gain bandwidth product of the amplifier. The sensitivity of the amplifier to output voltage nonlinearities cannot be arbitrarily reduced because of the limitation of the gain bandwidth due to instability and oscillation. These output stage nonlinearities are found in both bipolar and MOSFET designs. The output device capacitances require currents to charge them to follow the nonlinear voltage and result in nonlinear currents. The effect of the currents can be reduced by increasing the nominal current gains of the intermediate and output stages.
These output stage nonlinearities will be coupled through the Miller compensation capacitors into the input stage. The compensation capacitors will differentiate the distorted voltage into currents and couple these in such a way as to affect the intermediate stage, and thus require the input stage to have a distorted voltage waveforms at its input.
Another capacitance commonly found in power amplifier designs is a high frequency bypass capacitor connecting the intermediate stage to the input stage. This capacitor helps stability by shunting the high frequencies directly to the input stage, bypassing the output stage and its pole. It also permits an optimum Miller compensation capacitor to be used. Unfortunately, this Miller compensation capacitor and the bypass capacitor are of such sizes that they couple nonlinearities in voltage from the output stage back to the input stage, and the Miller capacitor cannot be arbitrarily reduced without causing instability in the amplifier. The problem then is to reduce coupling of output voltage nonlinearities through the compensation and bypass capacitors to the input.
This problem can be solved in three ways: by effectively fooling the amplifier into thinking the gain-bandwidth product is greater than it is; move the compensation capacitors to the output of the output stage; and reduce the output stage nonlinearity itself. Cherry recommends using nested differentiating feedback loops as a solution. Such loops increase the apparent gain-bandwidth product at frequencies less than the actual cutoff frequency. Also the Miller compensation capacitor is connected between the input of the intermediate stage input and the output of the output stage. This reduces the effect of the output stage nonlinearity.
The topology recommended by Cherry is stable with resistive loads. However, it has two disadvantages: Miller-effect (internal device) capacitance in the intermediate stage is still present to produce some coupling of the output stage nonlinearity to the input stage, and a bypass capacitor from the intermediate stage to the input stage cannot be used to improve stability without further coupling the output stage nonlinearity to the input stage. The ultimate solution lies in reducing the output stage voltage nonlinearity, but that is sometimes difficult. It would be better to isolate the nonlinearities of the output stage from the intermediate stage, or whatever serves as the signal source for the output stage, in order not to couple nonlinearities in the output stage into the signal source.